This invention relates to improved method and apparatus for decomposing water and other liquids by photoelectrolysis. It relates more particularly to the solar production of hydrogen and oxygen gases from water.
Hydrogen gas may be produced from water and sunlight in different ways. Because hydrogen is a versatile, storable, transportable, clean, and non-polluting fuel, production of that gas from these renewable sources is very desirable. The same is true of oxygen to a lesser extent. However, none of the proposed solar methods of producing these gases has yet proven to be practical from a commercial standpoint.
The methods for solar water splitting may be divided roughly into two classes; namely thermal and photoelectrochemical. The thermal methods involve the use of concentrated sunlight to produce high temperature heat, which is then used to separate water into hydrogen and oxygen. One thermal method of which we are aware heats water vapor to dissociation at very high temperatures (E. Bilgen, Int. J. Hydrogen Energy, Vol. 9, p. 53 , 1984). Another approach is to operate several endothermic chemical reactions cyclically with concentrated solar heat to split the water into its elements (T. Ohta and I. Abe, Int. J. Hydrogen Energy, Vol. 10, p. 275, 1985). Also, of course, solar heat can be used to generate steam to operate a conventional electric generator whose output then powers a separate water electrolysis cell. All of these thermal methods require expensive mirror and tracking systems to concentrate the sunlight.
The photoelectrochemical approach to solar water splitting, on the other hand, uses the sunlight to produce directly an electric current which then electrolyzes the water into its elements. One such method uses conventional photovoltaic cells that convert sunlight into electricity which is then used to electrolyze the water. Actually, several photovoltaic cells must be wired together in series to produce sufficient voltage for each electrolytic cell. This is because each photocell produces less than 1 volt, while each electrolytic cell usually operates at a voltage of about 2 volts so that an economical electrode area can be used in each electrolytic cell. Actually, commercial water electrolyzers usually connect many electrolytic cells in series in order to combine them efficiently in one high-pressure electrolyte compartment. Consequently, a large number of solar cells must be connected electrically in series by external circuitry in order to run these high-voltage electrolysis systems. If any one of these solar cells fails, or is shadowed from sunlight, the electrolytic action stops in the entire system and the production of hydrogen and oxygen stops entirely.
A second photoelectrochemical method places an illuminated semiconducting electrode in direct contact with an aqueous electrolyte solution. Light absorbed in the semiconductor causes direct reaction of the minority carriers in the semiconductor with the electrolyte at the surface of the semiconductor. Photo-anodes of n-TiO.sub.2 release oxygen (A. Fujishima and K. Honda, Nature, Vol. 238, p. 37, 1972), while photocathodes of p-GaP (A.J. Nozik, Applied Physics Letters, Vol. 30, p. 567, 1977) and p-InP (E. Aharon-Shalom and A. Heller, J. Electrochem. Soc., Vol. 129, p. 2865, 1982), have been used to generate hydrogen gas. This type of photoelectrolyzer is disadvantaged because evolving hydrogen and oxygen gases tend to recombine thereby lowering the efficiency of the apparatus appreciably. Devices of this type are disadvantaged because they are quite difficult to make and/or they must be illuminated from both sides in order to operate properly.